As is known, there are a variety of types of magnetic field sensing elements, including, but not limited to, Hall effect elements, magnetoresistance elements, and magnetotransistors. As is also known, there are different types of Hall effect elements, for example, a planar Hall element, and a vertical Hall element. As is also known, there are different types of magnetoresistance elements, for example, a giant magnetoresistance (GMR) element, an anisotropic magnetoresistance element (AMR), a tunneling magnetoresistance (TMR) element, and a magnetic tunnel junction (MTJ).
Hall effect elements generate an output voltage proportional to a magnetic field. In contrast, magnetoresistance elements change resistance in proportion to a magnetic field. In a circuit, an electrical current can be directed through the magnetoresistance element, thereby generating a voltage output signal proportional to the magnetic field.
Magnetoresistance elements tend to have a hysteresis characteristic with regard to resistance versus applied magnetic field. Different types of magnetoresistance elements can have different hysteresis characteristics, some of which are described below.
The hysteresis characteristic can result in an uncertainty as to a magnetic field being sensed by the magnetoresistance element. In other words, at different times, the magnetoresistance element can have two different resistances when exposed to the same external magnetic field.
Magnetic field sensors, i.e., circuits that use magnetic field sensing elements, are used in a variety of applications, including, but not limited to, a current sensor that senses a magnetic field generated by a current flowing in a current-carrying conductor, a magnetic switch that senses the proximity of a ferromagnetic object, a rotation detector that senses passing ferromagnetic articles, for example magnetic domains of a ring magnet, and a magnetic field sensor that senses a magnetic field density of a magnetic field.
As is known in the art, a conventional magnetic field sensor, e.g., a current sensor, can be arranged in either an open loop or a closed loop configuration. An “open loop” current sensor includes a magnetic field sensing element in proximity to a current-carrying, or primary, conductor. The current sensor provides an output signal proportional to the magnetic field generated by current passing through the primary conductor.
A “closed loop” current sensor additionally includes a secondary conductor in proximity to the magnetic field sensing element. A current is passed through the secondary conductor in order to generate a magnetic field that opposes and cancels the magnetic field generated by a current passing through the primary conductor. Thus, the magnetic field in the vicinity of the magnetic field sensing element is substantially zero. The current passed through the secondary conductor is proportional to the magnetic field in the primary conductor and thus, to the primary current. The closed loop configuration generally provides improved accuracy over the open loop configuration. This is because the effect of the above-described hysteresis characteristic associated with the magnetic field sensing element, and in particular, associated with a magnetoresistance element, is eliminated in many situations by maintaining the magnetic field on the magnetic field sensing element to be approximately zero. The closed loop configuration also generally provides improved linearity in comparison with the open loop configuration, as well as increased dynamic range.
Though the closed loop arrangement can eliminate the effect of the hysteresis characteristic in many situations, the closed loop arrangement is unable to eliminate the effect of the hysteresis characteristic in some situations. For example, if the magnetic field experienced by the magnetic field sensing element is very large, and large enough that the closed loop arrangement is unable to fully oppose the magnetic field generated by current carried by the primary conductor, then the magnetic field sensing element, and in particular, the magnetoresistance element, may saturate, and the hysteresis characteristic may thereafter result in the above-described uncertainty of the sensed magnetic field.
Furthermore, the above-described closed loop arrangement includes circuit components, for example, amplifiers, which have delay times. Therefore, due to the time delay, a rapid transient in the magnetic field experience by the magnetic field sensing element may not be fully opposed by the feedback circuit. Thus, in this situation also, the magnetic field sensing element, and in particular, the magnetoresistance element, may saturate, and the hysteresis characteristic may thereafter result in the above-described uncertainty of the sensed magnetic field.
Various parameters characterize the performance of magnetic field sensors, e.g., current sensors, including sensitivity and linearity. Sensitivity is related to a change in the resistance of the magnetoresistance element or the change in output voltage from the Hall effect element in response to a change in magnetic field. Linearity is related to the degree to which the resistance of the magnetoresistance element or the output voltage from the Hall effect element varies in direct proportion to the magnetic field.
In general, it is known that many types of magnetoresistance elements have a greater sensitivity than a Hall effect element. However, it is also known that many types of magnetoresistance elements have a hysteresis characteristic that can result in a greater uncertainty in a sensed magnetic field than a Hall effect element.
It would be desirable to provide a sensor (e.g., a current sensor) using a magnetoresistance element, and therefore, capable of a relatively high sensitivity, but for which the uncertainty of the sensed magnetic field (e.g., of the sensed current) due to a hysteresis characteristic is reduced.
While current sensors using magnetoresistance elements are described as examples above, it will be appreciated that conventional external magnetic field sensors and also conventional electrical signal isolators that use magnetoresistance elements suffer from the same disadvantages due to the hysteresis characteristic. It would, therefore, also be desirable to provide an external magnetic field sensor and also an electrical signal isolator using a respective magnetoresistance element, and therefore, capable of a relatively high sensitivity, but for which the uncertainty of the sensed magnetic field due to a hysteresis characteristic is reduced.